use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

use futures_core::ready;
use pin_project_lite::pin_project;
use tower_service::Service;

// Vendored from tower::util to reduce dependencies, the code is small enough.

// Not really pub, but used in a trait for bounds
pin_project! {
		#[project = OneshotProj]
		#[derive(Debug)]
		pub enum Oneshot<S: Service<Req>, Req> {
				NotReady {
						svc: S,
						req: Option<Req>,
				},
				Called {
						#[pin]
						fut: S::Future,
				},
				Done,
		}
}

impl<S, Req> Oneshot<S, Req>
where
	S: Service<Req>,
{
	pub(crate) const fn new(svc: S, req: Req) -> Self {
		Oneshot::NotReady {
			svc,
			req: Some(req),
		}
	}
}

impl<S, Req> Future for Oneshot<S, Req>
where
	S: Service<Req>,
{
	type Output = Result<S::Response, S::Error>;

	fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
		loop {
			let this = self.as_mut().project();
			match this {
				OneshotProj::NotReady { svc, req } => {
					ready!(svc.poll_ready(cx))?;
					let fut = svc.call(req.take().expect("already called"));
					self.set(Oneshot::Called { fut });
				},
				OneshotProj::Called { fut } => {
					let res = ready!(fut.poll(cx))?;
					self.set(Oneshot::Done);
					return Poll::Ready(Ok(res));
				},
				OneshotProj::Done => panic!("polled after complete"),
			}
		}
	}
}
